Rigid vinyl polymer compositions, such as polyvinyl chloride compositions, are utilized for producing a variety of structural articles such as plastic pipe, siding, containers, and sheets. These rigid compositions are substantially unplasticized.
In order to stabilize vinyl polymer against the decomposing influence of heat and light, basic lead compounds such as tribasic lead sulfate, dibasic lead phosphite, or dibasic lead phosphite sulfite are used. To enhance the effect of these lead compounds, typically metal soaps such as neutral or basic lead stearate and/or calcium stearate are added.
Lubricants are also added to vinyl polymers to facilitate the extrusion or other melt processing of the structural articles produced. Lubricants are generally classified as external or internal lubricants. An external lubricant provides a lubricating layer between the plastic melt and the metallic surfaces of the processing equipment. The external lubricant serves to coat the individual particles of the polymeric resin and inhibits their adherence to the metallic surfaces. In contrast, an internal lubricant reduces the effective melt viscosity of the vinyl polymer at the processing temperatures in order to improve its flow properties during processing as well as to promote fusion. An internal lubricant is generally needed only for thin extrusions such as films and thin-walled pipe.
The suitability of lubricants for vinyl polymer is determined by the type of stabilizer used in the vinyl polymer. For example, the PLASTICS ADDITIVE HANDBOOK, 3rd Edition (Hanser Publishers 1990) teaches that calcium stearate, stearic acid, hydroxystearic acid, unoxidized polyethylene waxes, paraffin waxes, microwaxes, and FT-waxes (synthetic wax produced by Fischer Trosch process) are suitable lubricants for lead stabilized polyvinyl chloride thick-walled pipes.
As discussed by Kosior et al., "Processing Aids: The Effect of Polymethyl Methacrylate on the Fusion of Rigid Poly(vinyl chloride)", British Polymer Journal 18(2), 94 (1986), lubricants, in addition to preventing adhesion to metal surfaces, have been shown to retard the progress of fusion of polyvinyl chloride. Processing aids based on polymethyl methacrylate are effective in accelerating the rate of fusion of polyvinyl chloride and are currently used in such compositions. For example, Kosior et al. teach a lead stabilized polyvinyl chloride which used a polymethyl methacrylate processing aid. See also Cogswell, "Influence of Acrylic Processing Aids on the Rheology and Structure of Polyvinyl Chloride", Pure & Appl. Chem, 55(1), 177 (1983).
Known lead stabilized rigid polyvinyl chloride pipe formulations include compositions such as:
______________________________________ Resin PVC 100 phr Pigment Titanium Dioxide 1.0 Filler Coated CaCO.sub.3 5.0 Stabilizer Tribasic lead sulfate 1.0 Co-Stabilizer Lead stearate 0.8 Co-Stabilizer/ Calcium Stearate 0.4 Lubricant Internal Lubricant Steryl Stearate 0.6 External Lubricant A-C .RTM. 6A (unoxidized 0.2 homopolymer polyethylene wax, Brookfield viscosity at 140.degree. C. of 350 cps) External Lubricant A-C .RTM. 629 (oxidized polyethylene 0.15 wax, Brookfield viscosity at 140.degree. C. of 200 cps) ______________________________________
which is disclosed in TECHNICAL DATA FOR LOW MOLECULAR WEIGHT POLYETHYLENES AND DERIVATIVES by AlliedSignal Inc., a chapter of the POLYMER ADDITIVE HANDBOOK (1992);
______________________________________ Rigid 4-Cell Resin 100 phr Lead Stabilizer 5.0 Calcium Carbonate 1.5 Titanium Dioxide 2.0 Acrylic Processing Aid 3.0 Calcium Stearate 1.0 A-C .RTM. 629A (oxidized polyethylene 0 to 1 wax, Brookfield viscosity at 140.degree. C. of 200 centipoises) ______________________________________
which is disclosed by David Hurwitz, "The Use of Low Molecular Weight Polyethylene in Rigid PVC Lubrication", Society of Plastics Engineers, 31st Annual Technical Conference, 349 (May 1973), and
______________________________________ PVC 100 phr Tribasic Lead Sulfate 0.5 Dibasic Lead Stearate 1.0 Calcium Stearate 0.4 Stearic Acid 0.4 A-C .RTM. 617A (unoxidized 0.2-0.4 homopolymer polyethylene wax, Brookfield viscosity at 140.degree. C. of 180 centipoises) ______________________________________
which is disclosed in A-C.RTM. POLYETHYLENES FOR PVC by AlliedSignal Inc. (1986). See also Technical Data on A-C.RTM. Polyethylenes and Copolymers for Plastics by AlliedSignal Inc. (1973) which teaches that A-C.RTM. 629A (low density oxidized polyethylene wax) is useful for lead stabilized polyvinyl chloride.
Additives which speed fusion in polyvinyl chloride compositions are desired in the art. ASTM D 2538 defines fusion time as the time froth the point of loading the composition into the torque rheometer to the point of maximum torque. See also Robert A. Lindner, "External Lubricants that Speed Fusion", Plastics Compounding (September/October 1989). The first and third formulations above are undesirable because the unoxidized homopolymer polyethylene wax having a Brookfield viscosity at 140.degree. C. of 180 or 350 centipoises delays fusion and thus, increases fusion time. Additionally, the unoxidized homopolymer polyethylene wax serves as more of a fusion control rather than an external lubricant and thus, it is necessary to add an additional external lubricant such as oxidized polyethylene wax having a Brookfield viscosity at 140.degree. C. of 200 centipoises or stearic acid to the composition. Although the first and third formulations above have good lubricity, the fusion times are too long. Additionally, stearic acid has a low melting point which means that it evaporates during normal processing conditions.
The second formulation above is undesirable because a processing aid is used which increases the cost of the final composition. Also, the presence of an acrylic processing aid in the final product may cause high viscosity and poor flow.
U.S. Pat. No. 4,203,880 discloses tin stabilized polyvinyl chloride having a lubricant package comprising oxidized polyethylene wax and a known external lubricant such as paraffin oils, paraffin waxes, liquid and solid hydrocarbons, unoxidized polyethylene waxes, montan ester waxes, lead stearate, mineral oil, 12-hydroxystearic acid, ethylene bis-stearamide, and glycol esters of fatty acids that contain 10 to 20 or more carbon atoms and cites two articles which teach the preceding known external lubricants: Illmann, "Waxes as Lubricants in Plastics Processing", SPE Journal, 71 (June 1967) and King et al., "Characterization of Lubricants for Polyvinyl Chloride", Polymer Engineering and Science 12(2), 112 (March 1972).
U.S. Pat. No. 4,203,880 also discloses that other heat stabilizers useful with polyvinyl chloride include organosulfur-containing antimony compounds and alkaline earth metal salts of carboxylic acids. The reference further states that the known external lubricants and oxidized polyethylene waxes should be compatible with most, if not all, of the conventional heat stabilizers used for polyvinyl chloride. However, Illmann, "Waxes as Lubricants in Plastics Processing", SPE Journal, 71 (June 1967) and King et al., "Characterization of Lubricants for Polyvinyl Chloride", Polymer Engineering and Science 12(2), 112 (March 1972) only teach tin, calcium/zinc, and barium/cadmium stabilizers in combination with the known external lubricants. Additionally, although the PLASTICS ADDITIVE HANDBOOK 3rd Edition (Hanser Publishers 1990) teaches that oxidized polyethylene waxes are suitable for tin stabilized thick-walled polyvinyl chloride, the reference does not teach that oxidized polyethylene waxes are suitable for lead stabilized thick-walled polyvinyl chloride.
It would be desirable to have a lead stabilized vinyl polymer composition which has improved fusion time, i.e. reduced fusion time. It would also be desirable to have a lead stabilized vinyl polymer composition which has improved fusion time, i.e. reduced fusion time, in the absence of a resinous processing aid such as an acrylic processing aid.